In high speed web-offset printing presses, ink is applied to both surfaces of the web substantially simultaneously and the ink must be dried before the web can be brought into contact with a solid surface. To dry the ink, the printed or coated web is passed through a float or floater type oven wherein the web is held under tension and floated between cushions of hot air while the ink is being dried. Upon leaving the oven, the web must be cooled before further processing, for example, sheeting, folding or rewinding. For the purpose, the web upon exiting the oven is passed over one or more chill drums or rolls, i.e., internally cooled drums, which reduce the web temperature from about 200.degree. F. to about 90.degree. F.
In moving the printed web through the oven, it is necessary to maintain the web under lengthwise or machine direction tension in order to cause the web to move through the oven without contacting any of the oven's surfaces. The heat and tension forces applied to the web in the oven cause the paper to contract on itself in the width-wise or cross machine direction, i.e., transversely of the length of the web, in a series of longitudinally extending hills and valleys or corrugations. If these become locked into the printed sheet, due for example to thermosetting of a thermosetting ink and/or a thermosetting coating on the paper, they frequently result in an unacceptable printed product which must be discarded and disposed of as waste.
Fluting is the term most often used to describe the permanent, cross-direction (CD), ridge-and-valley undulations created in light-weight coated (LWC) papers during lithographic web-offset printing. The phenomenon, which has been of practical concern for many years, is also referred to as corrugating, ridging, troughing, and waffling. Flutes are tension wrinkles created in the dryer and locked into the web upon drying of the ink and paper coating. The shapes of these flutes approximate sine waves with wavelengths in the range of 0.5 to 2.0 cm (0.2 to 0.8 inch). A line, drawn along a flute peak (or trough) runs, almost exactly, in the machine direction (MD). These aligned, regularly spaced corrugations detract from the quality of the printed image in a more aesthetically disturbing manner than would randomly distributed deformations of the same amplitude. The flutes form in the oven or dryer section of the web-offset press. Both web tension and above room temperature drying create the flutes. Ink coverage plays an important role; the heavily inked areas are more fluted, especially if they are printed on both sides. Coated papers exhibit severe fluting, and the higher the ratio of coating to fibre basis weight the worse will be the fluting. Light-weight sheets are more at risk; fluting is rarely observed when the basis weight is greater than about 120 grams per square meter (120 g/m.sup.2)(80 lbs/per 3300 sq. ft. ream). The wavelength of the flutes increases as sheet basis weight increases. Fluting occurs in hot-air impingement dryers. It does not occur when the ink is cured with ultraviolet radiation.
U.S. Pat. No. 3,442,211, No. 4,462,169 and No. 5,275,103 propose solutions to the problem of streaking of the printed web due to ink solvent condensation on the chill roll or rolls, but they do not address the problem of fluting or corrugating.
German patent publication DE 3022557 discloses a web fed printing machine for offset, gravure and flexographic printing comprising a printing station, a drying station and a cooling station, and including in the cooling station a chill roll having a concave peripheral surface for reducing the formation of folds at the side edges of the web. Folds occur on chill rolls when an excessively slack area in a paper web, usually near its edges, folds over and is pressed in place. This is a random phenomenon, usually corrected by optimum web tensions, and is not to be confused with fluting which is systematic and repetitive in nature.